ApplyCommand: add a base-85 codec

Add an implementation for base-85 encoding and decoding [1]. Git binary
patches use this format.

Base-85 encoding assembles bytes as 32-bit MSB values, then converts
these values to base-85 numbers (always 5 bytes) encoded as printable
ASCII characters. Decoding base-85 is the reverse operation. Note
that decoding may overflow on invalid input as 85^5 > 2^32. Encodings
always have a length that is a multiple of 5. If input length is not
divisible by 4, padding bytes are (logically) added, which are ignored
when decoding. The encoding for n bytes has thus always exactly length
(n + 3) / 4 * 5 in integer arithmetic (truncating division).

Includes tests.

[1] https://datatracker.ietf.org/doc/html/rfc1924

Bug: 371725
Change-Id: Ib5b9a503cd62cf70e080a4fb38c8cd1eeeaebcfe
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

1 files changed, 195 insertions, 0 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/Base85.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/Base85.java
new file mode 100644
index 0000000000..54b7cfcaa7
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/Base85.java
@@ -0,0 +1,195 @@
+/*
+ * Copyright (C) 2021 Thomas Wolf <thomas.wolf@paranor.ch> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.util;
+
+import java.nio.charset.StandardCharsets;
+import java.text.MessageFormat;
+import java.util.Arrays;
+
+import org.eclipse.jgit.internal.JGitText;
+
+/**
+ * Base-85 encoder/decoder.
+ *
+ * @since 5.12
+ */
+public final class Base85 {
+
+	private static final byte[] ENCODE = ("0123456789" //$NON-NLS-1$
+			+ "ABCDEFGHIJKLMNOPQRSTUVWXYZ" //$NON-NLS-1$
+			+ "abcdefghijklmnopqrstuvwxyz" //$NON-NLS-1$
+			+ "!#$%&()*+-;<=>?@^_`{|}~") //$NON-NLS-1$
+					.getBytes(StandardCharsets.US_ASCII);
+
+	private static final int[] DECODE = new int[256];
+
+	static {
+		Arrays.fill(DECODE, -1);
+		for (int i = 0; i < ENCODE.length; i++) {
+			DECODE[ENCODE[i]] = i;
+		}
+	}
+
+	private Base85() {
+		// No instantiation
+	}
+
+	/**
+	 * Determines the length of the base-85 encoding for {@code rawLength}
+	 * bytes.
+	 *
+	 * @param rawLength
+	 *            number of bytes to encode
+	 * @return number of bytes needed for the base-85 encoding of
+	 *         {@code rawLength} bytes
+	 */
+	public static int encodedLength(int rawLength) {
+		return (rawLength + 3) / 4 * 5;
+	}
+
+	/**
+	 * Encodes the given {@code data} in Base-85.
+	 *
+	 * @param data
+	 *            to encode
+	 * @return encoded data
+	 */
+	public static byte[] encode(byte[] data) {
+		return encode(data, 0, data.length);
+	}
+
+	/**
+	 * Encodes {@code length} bytes of {@code data} in Base-85, beginning at the
+	 * {@code start} index.
+	 *
+	 * @param data
+	 *            to encode
+	 * @param start
+	 *            index of the first byte to encode
+	 * @param length
+	 *            number of bytes to encode
+	 * @return encoded data
+	 */
+	public static byte[] encode(byte[] data, int start, int length) {
+		byte[] result = new byte[encodedLength(length)];
+		int end = start + length;
+		int in = start;
+		int out = 0;
+		while (in < end) {
+			// Accumulate remaining bytes MSB first as a 32bit value
+			long accumulator = ((long) (data[in++] & 0xFF)) << 24;
+			if (in < end) {
+				accumulator |= (data[in++] & 0xFF) << 16;
+				if (in < end) {
+					accumulator |= (data[in++] & 0xFF) << 8;
+					if (in < end) {
+						accumulator |= (data[in++] & 0xFF);
+					}
+				}
+			}
+			// Write the 32bit value in base-85 encoding, also MSB first
+			for (int i = 4; i >= 0; i--) {
+				result[out + i] = ENCODE[(int) (accumulator % 85)];
+				accumulator /= 85;
+			}
+			out += 5;
+		}
+		return result;
+	}
+
+	/**
+	 * Decodes the Base-85 {@code encoded} data into a byte array of
+	 * {@code expectedSize} bytes.
+	 *
+	 * @param encoded
+	 *            Base-85 encoded data
+	 * @param expectedSize
+	 *            of the result
+	 * @return the decoded bytes
+	 * @throws IllegalArgumentException
+	 *             if expectedSize doesn't match, the encoded data has a length
+	 *             that is not a multiple of 5, or there are invalid characters
+	 *             in the encoded data
+	 */
+	public static byte[] decode(byte[] encoded, int expectedSize) {
+		return decode(encoded, 0, encoded.length, expectedSize);
+	}
+
+	/**
+	 * Decodes {@code length} bytes of Base-85 {@code encoded} data, beginning
+	 * at the {@code start} index, into a byte array of {@code expectedSize}
+	 * bytes.
+	 *
+	 * @param encoded
+	 *            Base-85 encoded data
+	 * @param start
+	 *            index at which the data to decode starts in {@code encoded}
+	 * @param length
+	 *            of the Base-85 encoded data
+	 * @param expectedSize
+	 *            of the result
+	 * @return the decoded bytes
+	 * @throws IllegalArgumentException
+	 *             if expectedSize doesn't match, {@code length} is not a
+	 *             multiple of 5, or there are invalid characters in the encoded
+	 *             data
+	 */
+	public static byte[] decode(byte[] encoded, int start, int length,
+			int expectedSize) {
+		if (length % 5 != 0) {
+			throw new IllegalArgumentException(JGitText.get().base85length);
+		}
+		byte[] result = new byte[expectedSize];
+		int end = start + length;
+		int in = start;
+		int out = 0;
+		while (in < end && out < expectedSize) {
+			// Accumulate 5 bytes, "MSB" first
+			long accumulator = 0;
+			for (int i = 4; i >= 0; i--) {
+				int val = DECODE[encoded[in++] & 0xFF];
+				if (val < 0) {
+					throw new IllegalArgumentException(MessageFormat.format(
+							JGitText.get().base85invalidChar,
+							Integer.toHexString(encoded[in - 1] & 0xFF)));
+				}
+				accumulator = accumulator * 85 + val;
+			}
+			if (accumulator > 0xFFFF_FFFFL) {
+				throw new IllegalArgumentException(
+						MessageFormat.format(JGitText.get().base85overflow,
+								Long.toHexString(accumulator)));
+			}
+			// Write remaining bytes, MSB first
+			result[out++] = (byte) (accumulator >>> 24);
+			if (out < expectedSize) {
+				result[out++] = (byte) (accumulator >>> 16);
+				if (out < expectedSize) {
+					result[out++] = (byte) (accumulator >>> 8);
+					if (out < expectedSize) {
+						result[out++] = (byte) accumulator;
+					}
+				}
+			}
+		}
+		// Should have exhausted 'in' and filled 'out' completely
+		if (in < end) {
+			throw new IllegalArgumentException(
+					MessageFormat.format(JGitText.get().base85tooLong,
+							Integer.valueOf(expectedSize)));
+		}
+		if (out < expectedSize) {
+			throw new IllegalArgumentException(
+					MessageFormat.format(JGitText.get().base85tooShort,
+							Integer.valueOf(expectedSize)));
+		}
+		return result;
+	}
+}